During the processing of silver halide photographic materials, silver is carried out of the photographic elements and into solutions, particularly the fix and bleach/fix solutions. Numerous devices have been developed for removing the silver from the spent solutions. One such device utilizes a container within which is positioned a metallic filler, e.g., U.S. Pat. No. 3,369,801. The container is connected to the photographic processing unit, resulting in the spent silver containing solution flowing completely through the container holding the metallic filler prior to the solution being sewered or returned to a fixing tank. While the solution is being circulated, the silver is deposited as a sludge in the container.
This type of system utilizes an electron exchange which occurs between the more and less noble metal in order to recover the silver in the spent solution. The less noble metal in this type of recovery unit is iron in the form of inexpensive steel wool. When the silver rich solution is passed over the iron, an electron exchange occurs wherein the iron metal Fe.degree. is oxidized to ferrous or ferric ion and the silver ion (Ag.sup.+) is reduced to silver metal (Ag.degree.). The silver precipitates from the solution with some unreacted or oxidized iron, to form the above-mentioned sludge.
The iron or steel wool sits in a water based solution during dormant periods of such a process, i.e., overnight shut-down. Present experience shows that iron in surprisingly large quantities is being brought into solution. There is more iron brought into solution than would be expected from the stoichiometry of the metal displacement silver recovery reaction. To combat this transfer of iron during these dormant periods, certain materials, such as those described in U.S. Pat. No. 4,035,181, when added to the silver rich solution, have been found to reduce the non-productive amounts of iron being brought into solution during these dormant periods.
Although these materials are effective for their intended purpose, special care has to be taken to ensure that excess quantities of inhibitor are not introduced that would tend to retard the desired metal ion exchange reactions once the processing was restarted. Methods of introduction in similar situations have included hand measuring and automatic metering. Addition of inhibitor by hand not only is inconvenient in an automatic process but allows for the introduction of operator error during the measuring. Automatic metering devices solve the above mentioned problems that result when inhibitor is introduced by hand, but such metering devices involve a more complicated apparatus that has a higher likelihood of mechanical failure.
Also automatic metering devices have been found costly from the standpoint of not only the initial purchase but also the maintenance of such a complicated apparatus. This is in contrast to the low cost associated with the device disclosed herein.
In the present invention, the inhibitor is released slowly by diffusion resulting in a small amount of inhibitor being delivered to the flowing stream during the active silver recovery period. The diffusion mechanism also maintains the necessary amount of inhibitor to protect the metallic filler, such as steel wool, during the quiescent period. This results in a considerably improved efficiency of utilization of inhibitor, reducing the amount of inhibitor required to well below 10% of the amount which would otherwise be necessary.